Natural Drainage Features

The key to preserving a natural drainage system for urbanizing areas is understanding the predevelopment water balance and designing to minimize interference with that system. The soil and hydrology of the site must be studied so that high-density, highly impervious locations, such as shopping centers and industrial complexes, are located in areas with low infiltration potential. Recharge areas should be preserved as open, undisturbed space in parks and woodlands. Runoff from developed areas should be directed to recharge areas and detained to use the full infiltration potential. Broad, grassy swales will slow runoff and maximize infiltration. The drainage plan can include variable depth detention ponds that rise during a runoff event and return to a base level during dry weather.

Realizing that the design goal is maximizing infiltration-recharge and minimizing runoff, the planner should incorporate the following techniques into a site plan:

• Roof leaders should discharge to pervious areas or seepage pits. Dry (French) wells, consisting of borings filled with gravel, can be used for infiltration of rooftop runoff.

• As much area as possible should be left in a natural, undisturbed state. Earthwork and construction traffic will compact soil and decrease infiltration.

• Steep slopes should be avoided. They contribute to erosion and lessen recharge.

• Large impervious areas should be avoided. Parking lots can be built in small units and drained to pervious areas.

• No development should be permitted in flood plains.

Porous pavement is an alternative to conventional pavement. (Thelen and Howe 1978; Dinitz 1980). It provides storage, enhancing soil infiltration to reduce surface and volume from an otherwise impervious area.

For parking lots and access roads, planners can use modular pavement systems. Pavers are placed on a prepared sand and gravel base, which overlays the subsoil. The voids of the pavers are filled with either sand, gravel, or sod. Frost problems are minimal.

FIG. 9.21.1 Interception and diversion measures.

TABLE 9.21.2 SELECTING BMPs BY POLLUTANTS

Methods of Control

Structural

Vegetative

Management

Sediment (TSS, cobble embeddedness, turbidity)

Control erosion on land and streambank

Route runoff through BMPs that capture sediment Dispose of sediment properly

Nutrients: N, P (nuisance algae,

Minimize sources

Uptake all that is applied to the land or contain and recycle/ reuse (dissolved form control—commercial nutrients) Contain animal waste, process and land apply, or export to a different watershed (dissolved form control—animal waste) Minimize soil erosion and sediment delivery (adsorbed form control)

Intercept, treat runoff before it reaches the water (suspended form control)

Pathogens (bacteria, viruses, etc.)

Minimize source

Minimize movement so bacteria dies

Treat water

Metals

Control soil sources

Terraces; diversions; grade stabilization structures; streambank protection and stabilization Sediment basins low dissolved oxygen, odor)

Animal waste system (lagoon, storage area); fences (livestock exclusion); diversions; terraces Terrace; tailwater pit; runoff retention pond; wetland development

Diversion; pit/pond/lagoon; compost facility

Terrace; diversion; stream-bank protection and stabilization; sediment pond; critical area treatment See 1-3; water treatment (filtration or flocculation) for high-value crops

Fences

Animal waste storage; detention pond

Waste treatment lagoon; filtration

Cover crops and rotations; conservation tillage; critical area planting

Filter strip; grassed waterway; stripcropping; field borders

Range management; crop rotations

Cover crop; strip cropping; riparian buffer zone; change crop or grass species to one that is more nutrient demanding See 2(a)

Conservation tillage; filter strip; riparian buffer zone; cover crop

Riparian buffer zone

Filter strips; riparian buffer zones

Artificial wetland/rock reed microbial filter

Crop/plant selection

Contour farming; riparian area protection; proper grazing use and range management

Beneficial use of sediment—wetland enhancement

Range and pasture management; proper stocking rate; waste composting; nutrient management

Recycle/reuse irrigation return flow and runoff water; nutrient management; irrigation water management

Lagoon pump out; proper irrigation management

Nutrient management

See four preceding items, this column

Animal waste management, especially proper application rate and timing Proper site selection for animal feeding facility; proper application rate of waste Recycle and reuse

Avoid adding materials containing trace metals

Continued

Methods of Control

Structural

Vegetative

Management

Control added sources

Tailwater pit; reuse/recycle

Crop selection

Irrigation water

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